Image capturing apparatus and image capturing system

ABSTRACT

An image capturing apparatus performs a global electronic shutter operation in which a plurality of pixels are exposed during the same exposure period. In a first period, charge is accumulated by a photoelectric conversion unit. In a second period, accumulation units of a plurality of pixels accumulate charge. A ratio of saturation charge quantity of the photoelectric conversion unit to saturation charge quantity of the accumulation unit has a certain relationship with a ratio of a length of the first period to the sum of the length of the first period and the length of the second period.

BACKGROUND

1. Technical Field

The present disclosure relates to an image capturing apparatus and animage capturing system.

2. Description of the Related Art

Use of global electronic shutters in CMOS image sensors is proposedrecently. An image capturing apparatus described in Japanese PatentLaid-Open No. 2004-111590 has an effect that an object image is notdistorted even when capturing images of a quickly moving object.

SUMMARY

An embodiment according to the present disclosure provides an imagecapturing apparatus. The image capturing apparatus includes a pluralityof pixels. Each of the plurality of pixels includes a photoelectricconversion unit configured to generate and accumulate charge in responseincident light, an accumulation unit configured to accumulate thecharge, an amplifier unit configured to output a signal based on thecharge, a first transfer switch configured to transfer the charge to theaccumulation unit from the photoelectric conversion unit, and a secondtransfer switch configured to transfer the charge to the amplifier unitfrom the accumulation unit. The image capturing apparatus includes anoutput line to which signals from the amplifier units of the pluralityof pixels are output. At a first time point, the photoelectricconversion units of the plurality of pixels start accumulation of thecharge. The first transfer switches of the plurality of pixels are keptOFF from the first time point to a second time point, and thephotoelectric conversion units of the plurality of pixels accumulatecharge generated in a first period from the first time point to thesecond time point. In a second period from the second time point to athird time, the accumulation units of the plurality of pixels accumulatecharge generated by the photoelectric conversion units in the firstperiod from the first time point to the second time point, and chargegenerated by the photoelectric conversion units in the second period. Atthe third time, the first transfer switches of the plurality of pixelsare controlled from ON to OFF. Saturation charge quantity A₁ (“A”followed by a suffix of “1”) of the photoelectric conversion unit,saturation charge quantity A₂ (“A” followed by a suffix of “2”) of theaccumulation unit, the first period P₁ (“P” followed by a suffix of“1”), and the second period P₂ (“P” followed by a suffix of “2”) satisfya certain relationship.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates equivalent circuits of pixels of an image capturingapparatus.

FIG. 2 is a diagram schematically illustrating a sectional structure ofan image capturing apparatus.

FIG. 3 is a diagram illustrating driving pulses of an image capturingapparatus.

FIG. 4 is a diagram illustrating driving pulses of an image capturingapparatus.

FIG. 5 is a diagram schematically illustrating an operation of an imagecapturing apparatus.

FIG. 6 is a diagram schematically illustrating a sectional structure ofan image capturing apparatus.

FIG. 7 is a diagram illustrating equivalent circuits of an imagecapturing apparatus.

FIG. 8 is a diagram schematically illustrating a sectional structure ofan image capturing apparatus.

FIGS. 9A and 9B are diagrams illustrating driving pulses of an imagecapturing apparatus.

FIG. 10 is a diagram schematically illustrating a sectional structure ofan image capturing apparatus.

FIG. 11 is a diagram schematically illustrating a sectional structure ofan image capturing apparatus.

FIG. 12 is a diagram illustrating driving pulses of an image capturingapparatus.

FIG. 13 is a diagram schematically illustrating an operation of an imagecapturing apparatus.

FIG. 14 is a block diagram illustrating a configuration of an imagecapturing system.

DESCRIPTION OF THE EMBODIMENTS

According to some embodiments, a pixel size may be reduced whileincreasing saturation charge quantity.

In the image capturing apparatus described in Japanese Patent Laid-OpenNo. 2004-111590, all the charge generated by photoelectric conversionfor obtaining one image or one frame is accumulated in a photoelectricconversion unit. Then, charge is transferred from the photoelectricconversion units to accumulation units simultaneously with all thepixels, and another photoelectric conversion for obtaining the nextimage or the next frame is started. Therefore, in order to increasesaturation charge quantity of pixels, saturation charge quantity of thephotoelectric conversion unit and saturation charge quantity of theaccumulation unit need to be substantially the same. If saturationcharge quantity of the photoelectric conversion unit increases, an areaof the photoelectric conversion unit also increases. Therefore, thepixel size may become large.

The inventers found that, in some image capturing apparatuses,increasing saturation charge quantity of pixels has been difficult.According to some embodiments, in an image capturing apparatus capableof performing a global electronic shutter, the pixel size may be reducedwhile increasing saturation charge quantity.

An embodiment provides an image capturing apparatus including aplurality of pixels and an output line to which signals from theplurality of pixels are output. Each of the plurality of pixels includesa photoelectric conversion unit, an accumulation unit configured toaccumulate charge, and an amplifier unit configured to output a signalbased on the charge generated by the photoelectric conversion unit. Eachof the pixels is further provided with a first transfer switchconfigured to transfer the charge to the accumulation unit from thephotoelectric conversion unit, and a second transfer switch configuredto transfer the charge to the amplifier unit from the accumulation unit.With this configuration, an image pickup operation in whichphotoelectric conversion periods coincide among a plurality of pixels,i.e., a global electronic shutter, may be performed. An electronicshutter is, for example, defined as to electrically control accumulationof charge generated in response to incident light.

In some embodiments according to the present disclosure, at first timepoint, the photoelectric conversion units of the plurality of pixelsstart accumulation of charge simultaneously. From the first time pointto, or until, second time point, the first transfer switches of theplurality of pixels are kept OFF. Charge generated in this period isaccumulated in the photoelectric conversion units. The period from thefirst time point to the second time point corresponds to a first period.In other words, the first period may be defined as stating at the firsttime point and ending at the second time point.

In a period from the second time point to third time, corresponding to asecond period, the accumulation units of the plurality of pixelsaccumulate charge. At this time, the accumulation units accumulate bothof charge generated in the first period and charge generated in thesecond period. At the third time, the first transfer switches of theplurality of pixels are controlled from ON to OFF simultaneously.

In some embodiments according to the present disclosure, a ratio ofsaturation charge quantity of the photoelectric conversion unit tosaturation charge quantity of the accumulation unit is substantiallyequal to a ratio of the length of the first period to the sum of thelength of the first period and a length of the second period.

The size of the photoelectric conversion unit and the size of theaccumulation unit may be reduced by driving these units in accordancewith their saturation charge quantity relation. With this configuration,a global electronic shutter may be performed while reducing the pixelsize and increasing saturation charge quantity.

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. Embodiments according to the presentdisclosure are not limited to those described below. For example,embodiments of the present disclosure also include: an example in whicha configuration of any of the following embodiments is partially addedto another embodiment; or an example in which a configuration of any ofthe following embodiments is replaced by a configuration of anotherembodiment. In the following embodiments, a first conductivity type isn-type and a second conductivity type is p-type. However, the firstconductivity type may be p-type and the second conductivity type may ben-type.

First Embodiment

A first embodiment is described. FIG. 1 illustrates equivalent circuitsof pixels of an image capturing apparatus. Only four pixels 20 areillustrated in FIG. 1, but the image capturing apparatus actuallyincludes more pixels.

Each pixel 20 is provided with a photoelectric conversion unit 1, anaccumulation unit 2, an amplifier unit 10, a first transfer switch 4,and a second transfer switch 5. The pixel 20 is further provided with areset transistor 9 and a selection transistor 7.

In the photoelectric conversion unit 1 generates charge in response toincident light. The photoelectric conversion unit 1 accumulates chargegenerated in response to incident light. The first transfer switch 4transfers the charge in the photoelectric conversion unit 1 to theaccumulation unit 2. The accumulation unit 2 accumulates the chargegenerated by the incident light at a place other than the photoelectricconversion unit 1. The second transfer switch 5 transfers the charge inthe accumulation unit 2 to an input node 3 of the amplifier unit 10. Thereset transistor 9 resets a voltage of the input node 3 of the amplifierunit 10. The selection transistor 7 selects the pixel 20 that outputs asignal to an output line 8. The amplifier unit 10 outputs a signal basedon the charge generated by the incident light to the output line 8. Theamplifier unit 10 is, for example, a source follower. The first transferswitch 4 and the second transfer switch 5 are MOS transistors.

A control line Tx1 is connected to the first transfer switch 4. Acontrol line Tx2 is connected to the second transfer switch 5. In thepresent embodiment, a plurality of pixels are arranged in a matrixpattern. A common control line is connected to pixels included in onerow. Here, for example, pixels on the n-th row are described as controlline Tx1(n).

With this configuration, the photoelectric conversion unit 1 mayaccumulate the charge that is generated while the accumulation unit 2accumulates the charge. Thus, an image pickup operation in whichphotoelectric conversion periods coincide among a plurality of pixels,i.e., a global electronic shutter, may be performed.

FIG. 2 schematically illustrates a sectional structure of an imagecapturing apparatus. FIG. 2 illustrates a cross section of a pixel.Components having the same functions as those of FIG. 1 are denoted bythe same reference numerals. FIG. 2 illustrates a front-sideillumination image capturing apparatus, but a back-side illuminationimage capturing apparatus may also be used.

The photoelectric conversion unit 1 has an embedded photodiodestructure. The photoelectric conversion unit 1 includes an n-typesemiconductor region 11 and a p-type semiconductor region 12. The n-typesemiconductor region 11 and the p-type semiconductor region 12constitute a p-n junction. The p-type semiconductor region 12 may reducenoise in interfaces.

The p-type semiconductor region 14 is a well. The n-type semiconductorregion 13 is disposed below the n-type semiconductor region 11. Impurityconcentration of the n-type semiconductor region 13 is lower than theimpurity concentration of the n-type semiconductor region 11. Thus,charge generated at a deep position is collected in the n-typesemiconductor region. Here, the n-type semiconductor region 13 may be ap-type semiconductor region. A p-type semiconductor region 17 thatbecomes a potential barrier to the charge is disposed below the n-typesemiconductor region 13.

The accumulation unit 2 includes an n-type semiconductor region 201.Charge that becomes a signal is accumulated in the n-type semiconductorregion 201. Impurity concentration of the n-type semiconductor region201 is higher than impurity concentration of the n-type semiconductorregion 11.

A gate electrode 40 forms a gate of the first transfer switch 4. A gateelectrode 50 forms a gate of the second transfer switch 5. A part of thegate electrode 40 is disposed above the n-type semiconductor region 201via gate dielectric film. By applying a negative voltage to the gateelectrode 40, a hole may be induced on a surface of the n-typesemiconductor region 201. Thus, noise generated in interfaces may bereduced.

The accumulation unit 2 is shielded by a shading unit 203. The shadingunit 203 is made of metal that does not easily transmit visible light,such as tungsten and aluminum. A color filter 100 and a microlens 101are disposed on an opening of the shading unit 203.

The photoelectric conversion unit 1 and the accumulation unit 2 aredisposed on a semiconductor substrate. In the present embodiment, anarea of orthogonal projection of the photoelectric conversion unit 1 toa surface parallel to a surface of the semiconductor substrate issmaller than an area of orthogonal projection of the accumulation unit 2to that surface. With this configuration, an effect that saturationcharge quantity of pixels may be increased while reducing noise isobtained.

In order to increase saturation charge quantity of pixels, it isdesirable that the accumulation unit 2 has large saturation chargequantity. Saturation charge quantity of the accumulation unit 2 may beincreased by increasing impurity concentration of the n-typesemiconductor region 201 of the accumulation unit 2, or by enlarging anarea of the n-type semiconductor region 201 in a plan view. However, ifimpurity concentration of the n-type semiconductor region 201 is high, aleakage current and the like becomes large, which may cause greaternoise. Therefore, saturation charge quantity may be increased whilereducing impurity concentration of the n-type semiconductor region 201by enlarging the area of the n-type semiconductor region 201 in a planview.

Thus, saturation charge quantity of pixels may be increased whilereducing noise by enlarging the area of the accumulation unit 2 in aplan view, i.e., the area of orthogonal projection of the accumulationunit 2. Then, the area of the photoelectric conversion unit 1 in a planview is relatively easy to be small, and it is difficult to increasesaturation charge quantity of the photoelectric conversion unit 1. Thus,an effect that saturation charge quantity of pixels may be maintainedbecomes more significant even if saturation charge quantity of thephotoelectric conversion unit 1 is small.

A method for driving the image capturing apparatus of the presentembodiment is described. FIG. 3 schematically illustrates driving pulsesused in the present embodiment. FIG. 3 illustrates driving pulsessupplied to the control line Tx1 of the first transfer switch 4 and tothe control line Tx2 of the second transfer switch 5 of pixels on then-th to the (n+2)th rows. A corresponding transistor or a correspondingswitch is turned ON when the driving pulses are at a high level. Acorresponding transistor or a corresponding switch is turned OFF whenthe driving pulses are at a low level. A control unit provided in theimage capturing apparatus supplies these driving pulses. A logicalcircuit, such as a shift register and an address decoder, is used as thecontrol unit.

A previous frame is exposed before time T1. “Exposure” means that chargegenerated during photoelectric conversion is accumulated or kept as asignal. Charge generated before time T1 is accumulated in theaccumulation unit 2. The exposure of the previous frame ends when thefirst transfer switch 4 of charge from the photoelectric conversion unit1 to the accumulation unit 2 is controlled from ON to OFF in all thepixels simultaneously (time T1 of FIG. 1).

At time T1, all the charge of the photoelectric conversion unit 1 istransferred to the accumulation unit. This means that the photoelectricconversion unit 1 becomes an initial state. Therefore, at time T1, thephotoelectric conversion units 1 of pixels in three rows startaccumulation of charge simultaneously. Thus, in the present embodiment,accumulation of charge by the photoelectric conversion units 1 isstarted when the first transfer switch 4 is turned OFF.

In a period from time T1 to time T2 in which a first period elapses, thefirst transfer switch 4 is kept OFF. In the present embodiment, thefirst transfer switches 4 of all the pixels are kept OFF. It is onlynecessary that, however, the first transfer switch 4 is kept OFF at atleast one pixel in the period from time T1 to time T2.

Time when the first period elapsed since time T1 corresponds to time T2.That is, a period from time T1 to time T2 corresponds to the firstperiod. In the first period, charge generated in the first period isaccumulated in the photoelectric conversion unit 1. In the first period,the accumulation unit 2 accumulates charge generated in the previousframe.

In the first period, charge in the accumulation unit 2 is readsequentially to the input node 3 of the amplifier unit 10. Specifically,when the second transfer switch 5 of the n-th row is turned ON, chargein the accumulation unit 2 of the pixels of the n-th row is transferredto the input node 3. A voltage of the input node 3 changes in accordancewith capacity of the input node 3 and quantity of the transferredcharge. A signal based on the voltage of the input node is output to theoutput line 8 by the amplifier unit 10. Next, the same operation isperformed to the pixels of the (n+1)th row. This operation is performedto each of the pixels on the first row to the last row. After reading onthe last pixel is performed, the first transfer switches 4 and thesecond transfer switches 5 of all the pixels are turned OFF.

The first transfer switches 4 are turned ON at time T2. Then, charge inthe photoelectric conversion unit 1 is transferred to the accumulationunit 2. That is, charge generated in the first period is accumulated inthe accumulation unit 2 after time T2. In the present embodiment, thefirst transfer switches 4 of all the pixels are shifted ON from OFFsimultaneously. It is only necessary that, however, the first transferswitches 4 of a plurality of pixels are turned ON by time T2, and thetimings at which the pixels are turned ON may differ from each other.For example, beginning from the first transfer switch 4 of the pixel ofwhich the above-described reading operation is completed, the firsttransfer switch 4 may be turned ON.

Then, in a period from time T2 to time T3 in which a second periodelapses, the accumulation unit accumulates both charge generated in thefirst period and charge generated in the second period. In the presentembodiment, the first transfer switches 4 are kept ON in the secondperiod. Therefore, charge generated in the second period is immediatelytransferred to the accumulation unit 2. The period in which charge istransferred to the accumulation unit 2 from the photoelectric conversionunit 1 may be determined arbitrarily. The first transfer switches 4 maybe turned OFF at a part of the second period.

At time T3, the first transfer switches 4 of pixels of all the rows arecontrolled from ON to OFF simultaneously. Then, an exposure period ofone frame is completed. Thus, the exposure periods of all the pixels areidentical. That is, in all the pixels, exposure is started at time T1and completed at time T3. Exposure of a subsequent frame is started attime T3 and then operations in the period from time T1 to time T3 arerepeated thereafter.

Next, a reading operation of a signal from a single pixel is describedbriefly. FIG. 4 schematically illustrates driving pulses used in theimage capturing apparatus. FIG. 4 illustrates a driving pulse SELsupplied to the selection transistor 7, a driving pulse RES supplied tothe reset transistor 9, and a driving pulse Tx2 supplied to the secondtransfer switch 5. A corresponding transistor or a corresponding switchis turned ON when the driving pulses are at a high level. Acorresponding transistor or a corresponding switch is turned OFF whenthe driving pulses are at a low level.

In response to the driving pulses illustrated in FIG. 4, selection ofpixel, resetting, reading of a noise signal (N reading), transfer ofcharge, and reading of an optical signal (S reading) are performed. Theoutput signal may be AD converted outside the image capturing apparatus.Alternatively, the output signal may be AD converted inside the imagecapturing apparatus.

FIG. 5 schematically illustrates an operation of the image capturingapparatus. FIG. 5 illustrates an image pickup operation of from the n-thframe to the (n+1)th frame. An operation about the n-th frame isillustrated by a solid line and an operation about the (n+1)th frame isillustrated by a dotted line.

FIG. 5 illustrates an exposure period in each frame, a period in whichthe photoelectric conversion unit 1 accumulates charge, and a period inwhich the accumulation unit 2 accumulates charge. FIG. 5 illustratesthat reading operations of a plurality of pixels are performed in thefirst period. The reading operation in FIG. 5 is an operation includingtransfer of charge by the second transfer switch 5 and output of asignal by the amplifier unit 10 which are described with reference toFIGS. 3 and 4.

As illustrated in FIG. 5, immediately after the exposure of one frame iscompleted, the next exposure may be started. Therefore, since there issubstantially no period in which information is missing, image qualitymay be improved.

As illustrated in FIG. 5, in the first period in which the photoelectricconversion unit 1 is accumulating charge, the reading operation isperformed to each of a plurality of pixels. For this reason, even ifsaturation charge quantity of the photoelectric conversion unit 1 issmall, saturation charge quantity of pixels may be increased. Saturationcharge quantity of pixels is, for example, defined as the maximum valueof the amount of charge that is treated as a signal among chargegenerated in one exposure event, or a single frame. Saturation chargequantity of the photoelectric conversion unit 1 is, for example, definedas the maximum value of the amount of charge that is accumulated by thephotoelectric conversion unit 1, and saturation charge quantity of theaccumulation unit 2 is, for example, defined as the maximum value of theamount of charge that is accumulated by the accumulation unit 2.

The exposure period of one exposure event is the sum of the first periodand the second period. Here, charge of the previous frame accumulated inthe accumulation unit 2 is read in the first period. Thus, when thefirst period ends, the accumulation unit 2 may accumulate the charge.Therefore, it is only necessary that the photoelectric conversion unit 1may accumulate at least the charge generated in the first period. Sincethe amount of change generated in the first period is usually smallerthan the amount of charge generated in one exposure period, saturationcharge quantity of the photoelectric conversion unit 1 may be madesmall.

In the present embodiment, as illustrated in FIG. 5, the second periodin which the accumulation unit 2 accumulates the charge is longer thanthe first period. Therefore, saturation charge quantity of thephotoelectric conversion unit 1 may be made even smaller. However, thefirst period may be equal to or longer than the second period.

FIG. 5 illustrates an example in which the reading operation isperformed sequentially beginning from the first row. However, the orderin which the reading operation is performed is not limited to thisexample. It is only necessary that at least one reading operation isperformed to each of the pixels that constitute one frame in the firstperiod. In at least some of the pixels, a period after the accumulationunit 2 starts accumulation of charge in a certain frame until the sameaccumulation unit 2 starts accumulation of charge in a subsequent frameis equal to the exposure period.

It is desirable that a ratio of the first period to the sum of the firstperiod and the second period and the ratio of saturation charge quantityof the photoelectric conversion unit 1 to saturation charge quantity ofthe accumulation unit 2 are substantially equal. In particular, ifsaturation charge quantity of the photoelectric conversion unit 1 isdenoted by A₁, saturation charge quantity of the accumulation unit 2 isdenoted by A₂, the first period is denoted by P₁, and the second periodis denoted by P₂, A₁, A₂, P₁ and P₂ satisfy the relationship of thefollowing Expression (1). Here, the sum of the first period P₁ and thesecond period P₂ corresponds to one exposure period P₁+P₂.

$\begin{matrix}{\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} \leq \frac{P_{1} + {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}} & (1)\end{matrix}$

More desirably, a ratio of the first period to the sum of the firstperiod and the second period is equal to a ratio of saturation chargequantity of the photoelectric conversion unit 1 to saturation chargequantity of the accumulation unit 2. That is, A₁, A₂, P₁ and P₂ satisfya relationship of the following Expression (2):

$\begin{matrix}{\frac{A_{1}}{A_{2}} = {\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)}.}} & (2)\end{matrix}$

In the present embodiment, a ratio of one exposure period to the firstperiod is 4. That is, the first period is ¼ in length of one exposureperiod. For example, in a case in which a moving image is captured at 60frames per second, the exposure period is 1/60 second, and the firstperiod is 1/240 second.

Therefore, it is desirable that the ratio of saturation charge quantityof the photoelectric conversion unit 1 to saturation charge quantity ofthe accumulation unit 2 is close to ¼. This is because accumulation unit2 accumulates all of the charge generated in one exposure period whereasthe photoelectric conversion unit 1 only needs to accumulate ¼ in amountof that charge. Specifically, in a case in which saturation chargequantity of the accumulation unit 2 is 40000 electrons, it is desirablethat saturation charge quantity of the photoelectric conversion unit 1is equal to or greater than 5000 electrons and is equal to or smallerthan 25000 electrons. Preferably, saturation charge quantity of thephotoelectric conversion unit 1 is 10000 electrons.

By setting the ratio of saturation charge quantity as expressed byExpression (1), the optimum size of the photoelectric conversion unit 1and the accumulation unit 2 may be determined.

In a case in which charge overflowed from the photoelectric conversionunit 1 is to be accumulated by the accumulation unit 2, it is desirablethat A₁, A₂, P₁ and P₂ satisfy the following Expression (3):

$\begin{matrix}{\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} < {\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)}.}} & (3)\end{matrix}$

Specifically, in a case in which saturation charge quantity of theaccumulation unit 2 is 40000 electrons, saturation charge quantity ofphotoelectric conversion unit 1 is equal to or greater than 5000electrons and equal to or smaller than 10000 electrons. With thisconfiguration, since the charge overflowed from the photoelectricconversion unit 1 may be accumulated by the accumulation unit 2, mixingof charge may be reduced.

If A₁, A₂, P₁ and P₂ satisfy the following Expression (4), thephotoelectric conversion unit 1 may have a greater amount of saturationcharge quantity. Therefore, overflowing of charge may be reduced.

$\begin{matrix}{\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)} < \frac{A_{1}}{A_{2}} \leq \frac{P_{1} + {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}} & (4)\end{matrix}$

The image capturing apparatus of the present embodiment may have anoperation mode for performing a rolling shutter. In the operation modeof a rolling shutter, accumulation of charge by the photoelectricconversion units 1 of a plurality of pixels is started sequentially.Then, the first transfer switches 4 of a plurality of pixels aresequentially turned ON. Alternatively, the image capturing apparatus mayhave an operation mode for performing a global electronic shutter ofanother scheme. In the global electronic shutter of another scheme, aperiod in which the photoelectric conversion unit 1 accumulates chargeis equal to the exposure period.

As described above, according to the image capturing apparatus of thepresent embodiment, a global electronic shutter may be performed whileincreasing saturation charge quantity.

Second Embodiment

Another embodiment is described. The present embodiment differs from thefirst embodiment in the configuration of the accumulation unit. Thus,only a difference from the first embodiment is described and descriptionof the same configuration as that of the first embodiment is omitted.

Equivalent circuits of the present embodiment are the same as those ofthe first embodiment. FIG. 1 illustrates equivalent circuits of pixelsof the image capturing apparatus of the present embodiment. Descriptionabout FIG. 1 is the same as that of the first embodiment and is thusomitted.

A driving method of the present embodiment is the same as that of thefirst embodiment. That is, FIGS. 3 and 4 schematically illustratedriving pulses used in the present embodiment. FIG. 5 schematicallyillustrates an operation of the image capturing apparatus of the presentembodiment. Description about FIGS. 3 to 5 is the same as that of thefirst embodiment and is thus omitted.

FIG. 6 schematically illustrates a sectional structure of the imagecapturing apparatus. FIG. 6 illustrates a cross section of a pixel.Components having the same functions as those of FIGS. 1 to 5 aredenoted by the same reference numerals.

An accumulation unit 2 includes an n-type semiconductor region 201 and ap-type semiconductor region 202. The p-type semiconductor region 202 isdisposed above the n-type semiconductor region 201. The p-typesemiconductor region 202 may reduce noise in interfaces.

A gate electrode 40 of a first transfer switch 4 does not extend ontothe n-type semiconductor region 201. Thus, restriction on the layout isdecreased, and a degree of freedom of design may be increased.

As described above, according to the present embodiment, in addition tothe effect of the first embodiment, noise may be reduced.

Third Embodiment

Still another embodiment is described. The present embodiment differsfrom the first embodiment and the second embodiment in that a pixel isprovided with a discharge switch. Thus, only a difference from the firstembodiment and the second embodiment is described and description of thesame configuration as that of the first embodiment or the secondembodiment is omitted.

FIG. 7 illustrates equivalent circuits of pixels of an image capturingapparatus. The same components as those of FIG. 1 are denoted by thesame reference numerals. To make the drawing simple, the referencenumeral of a control line Tx1 and the reference numeral of a controlline Tx2 are omitted.

The control line Tx1 and the control line Tx2 are the same inconfiguration as those of the first embodiment. Each pixel is providedwith a discharge switch 18. The discharge switch 18 discharges charge inthe photoelectric conversion unit 1 to a power node, such as an overflowdrain. A control line OFG is connected to the discharge switch 18. Thedischarge switch 18 is, for example, a MOS transistor.

In the first embodiment, accumulation of charge by the photoelectricconversion unit 1 is started when the second transfer switch 5 iscontrolled from ON to OFF. In the present embodiment, as illustrated inFIGS. 9A and 9B, it is also possible to control the discharge switch 18to control the start of the exposure. Specifically, when the dischargeswitch 18 is controlled from ON to OFF, accumulation of charge by thephotoelectric conversion unit 1 is started. In this manner, exposuretime may be set arbitrarily.

FIG. 8 schematically illustrates a sectional structure of the imagecapturing apparatus. Components having the same functions as those ofFIGS. 1 and 2 are denoted by the same reference numerals. FIG. 8illustrates an example in which an accumulation unit 2 includes a p-typesemiconductor region 202 as in the second embodiment. The accumulationunit 2 does not necessarily include the p-type semiconductor region 202as in FIG. 1.

The discharge switch 18 is provided with an overflow control electrode16 and an overflow drain 15. Charge in the photoelectric conversion unit1 is discharged to the overflow drain 15 depending on a voltage suppliedto the overflow control electrode 16. A predetermined voltage issupplied to the overflow drain 15. The overflow control electrode 16 andthe overflow drain 15 are shielded by a shading unit 203.

A method for driving the image capturing apparatus of the presentembodiment is described. FIGS. 9A and 9B schematically illustratedriving pulses used in the present embodiment. FIGS. 9A and 9Billustrate driving pulses supplied to a control line Tx1, a control lineTx2, and a control line OFG of pixels on the n-th to the (n+2)th rows.The driving pulses supplied to the control line Tx1 and to the controlline Tx2 are the same as those in the first embodiment.

A corresponding transistor or a corresponding switch is turned ON whenthe driving pulses are at a high level. A corresponding transistor or acorresponding switch is turned OFF when the driving pulses are at a lowlevel. A control unit provided in the image capturing apparatus suppliesthese driving pulses. A logical circuit, such as a shift register and anaddress decoder, is used as the control unit.

FIGS. 9A and 9B are different in timing at which the discharge switch 18is operated. In FIG. 9A, the discharge switch 18 is controlled from ONto OFF at time T4. The generated charge is discharged while thedischarge switch 18 is kept ON. Therefore, according to the driving ofFIG. 9A, the exposure period corresponds to a period from time T4 totime T3. In FIG. 9B, the discharge switch 18 is controlled from ON toOFF at time T5. Therefore, according to the driving of FIG. 9B, theexposure period corresponds to a period from time T5 to time T3.

According to the present embodiment, the driving method may be changeddepending on the brightness of the object. For example, the drivingpulses illustrated in FIG. 3 are used at normal times, the drivingpulses illustrated in FIG. 9A are used when the object is bright, andthe driving pulses illustrated in FIG. 9B are used when the object iseven brighter.

In FIG. 9A, accumulation of charge by the photoelectric conversion unit1 is started at time T4. In a period from time T4 to time T3, thedischarge switch 18 is kept OFF.

A reading operation is performed in accordance with the driving pulsesillustrated in FIG. 4.

As described above, according to the present embodiment, in addition tothe effect of the first embodiment, the exposure time may be setarbitrarily.

Fourth Embodiment

Still another embodiment is described. The present embodiment differsfrom the first to the third embodiments in that a waveguide that guideslight to the photoelectric conversion unit is provided. Thus, only adifference from the first to the third embodiments is described anddescription of the same configuration as any of those of the first tothe third embodiments is omitted.

Equivalent circuits of the present embodiment are the same as those ofthe first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7illustrate equivalent circuits of pixels of an image capturing apparatusof the present embodiment. Since description about FIG. 1 is the same asthat of the first embodiment and description about FIG. 7 is the same asthat of the third embodiment, description of FIGS. 1 and 7 is omitted.

A driving method of the present embodiment is the same as that of thefirst embodiment or the third embodiment. That is, in a case in which apixel is not provided with a discharge switch, the driving pulsesillustrated in FIGS. 3 and 4 are used. In a case in which a pixel isprovided with a discharge switch, driving pulses illustrated in FIGS.9A, 9B and 4 are used. FIG. 5 schematically illustrates an operation ofthe image capturing apparatus of the present embodiment. Sincedescription about FIGS. 3 to 5 is the same as that of the firstembodiment and description about FIGS. 9A and 9B is the same as that ofthe third embodiment, description of FIGS. 3 to 5, 9A and 9B is omitted.

FIG. 10 schematically illustrates a sectional structure of the imagecapturing apparatus. The same components as those of FIG. 1, 2, 6, 7 or8 are denoted by the same reference numerals. FIG. 10 illustrates anexample in which, as in the second embodiment, an accumulation unit 2includes a p-type semiconductor region 202 and, as in the thirdembodiment, a pixel is provided with a discharge switch 18. However, thep-type semiconductor region 202 and the discharge switch 18 may beomitted.

In the present embodiment, a waveguide 301 is disposed to correspond tothe photoelectric conversion unit 1. The waveguide 301 guides incidentlight to the photoelectric conversion unit 1. Thus, sensitivity may beincreased. Especially, a decrease in sensitivity to obliquely incidentlight may be reduced.

A publicly known structure is used for the waveguide 301. In the presentembodiment, the waveguide 301 is formed by a material having arefractive index higher than that of surrounding insulating film. Forexample, the surrounding insulating film may be formed by interlayerinsulation film constituted by silicon oxide film, and the waveguide 301may be formed by silicon nitride film. Alternatively, a reflecting layeris provided on the periphery of the waveguide 301. The waveguide 301 maybe disposed to correspond to the photoelectric conversion units 1 of allthe pixels, or may be disposed to correspond only to the photoelectricconversion units 1 of some of the pixels.

An innerlayer lens 302 may be disposed between a color filter 100 andthe waveguide 301. The innerlayer lens 302 condenses light that haspassed the color filter 100 onto the waveguide 301. Sensitivity may beincreased by the innerlayer lens 302. Especially, a decrease insensitivity to obliquely incident light may be reduced.

As described above, according to the present embodiment, in addition tothe effect of the first embodiment, sensitivity may be increased.Especially in a case in which an area of the photoelectric conversionunit 1 in a plan view has been reduced to increase an area of theaccumulation unit 2 in a plan view, the effect of increasing sensitivityis significant.

Fifth Embodiment

Still another embodiment is described. The present embodiment differsfrom the first to the fourth embodiments in the configuration of theaccumulation unit. Thus, only a difference from the first to the fourthembodiments is described and description of the same configuration asany of those of the first to the fourth embodiments is omitted.

Equivalent circuits of the present embodiment are the same as those ofthe first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7illustrate equivalent circuits of pixels of an image capturing apparatusof the present embodiment. Since description about FIG. 1 is the same asthat of the first embodiment and description about FIG. 7 is the same asthat of the third embodiment, description of FIGS. 1 and 7 is omitted.

A driving method of the present embodiment is the same as that of thefirst embodiment or the third embodiment. That is, in a case in which apixel is not provided with a discharge switch, the driving pulsesillustrated in FIGS. 3 and 4 are used. In a case in which a pixel isprovided with a discharge switch, driving pulses illustrated in FIGS.9A, 9B and 4 are used. FIG. 5 schematically illustrates an operation ofthe image capturing apparatus of the present embodiment. Sincedescription about FIGS. 3 to 5 is the same as that of the firstembodiment and description about FIGS. 9A and 9B is the same as that ofthe third embodiment, description of FIGS. 3 to 5, 9A and 9B is omitted.

FIG. 11 schematically illustrates a sectional structure of the imagecapturing apparatus. The same components as those of FIG. 1, 2, 6, 7, 8or 10 are denoted by the same reference numerals. FIG. 11 illustrates anexample in which, as in the second embodiment, an accumulation unit 2includes a p-type semiconductor region 202 and, as in the thirdembodiment, a pixel is provided with a discharge switch 18. However, thep-type semiconductor region 202 and the discharge switch 18 may beomitted. FIG. 11 illustrates an example in which a waveguide 301 and aninnerlayer lens 302 are provided. However, the waveguide 301 and theinnerlayer lens 302 may be omitted.

In the present embodiment, below the n-type semiconductor region 201that is included in the accumulation unit 2 and accumulates charge, ap-type semiconductor region 303 and a p-type semiconductor region 304are disposed. The p-type semiconductor region 304 is disposed below thep-type semiconductor region 303. Impurity concentration of the p-typesemiconductor region 303 is higher than impurity concentration of thep-type semiconductor region 304. With this configuration, ingress of thecharge located at a deep portion of a substrate into the n-typesemiconductor region 201 may be prevented. Therefore, noise may bereduced.

In the present embodiment, the p-type semiconductor region 304 extendsto reach a p-type semiconductor region 17. With this configuration,mixing of colors of charge among pixels may be reduced.

As described above, according to the present embodiment, in addition tothe effect of the first embodiment, noise may be reduced.

Sixth Embodiment

Still another embodiment is described. The present embodiment differsfrom the first to the fifth embodiments in the driving method. Thus,only a difference from the first to the fifth embodiments is describedand description of the same configuration as any of those of the firstto the fifth embodiments is omitted.

Equivalent circuits of the present embodiment are the same as those ofthe first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7illustrate equivalent circuits of pixels of an image capturing apparatusof the present embodiment. Since description about FIG. 1 is the same asthat of the first embodiment and description about FIG. 7 is the same asthat of the third embodiment, description of FIGS. 1 and 7 is omitted.

A sectional structure of a pixel of the present embodiment is the sameas those of the first to the fifth embodiments. That is, FIGS. 2, 6, 8,10 and 11 schematically illustrate a sectional structure of a pixel ofthe present embodiment.

A method for driving the image capturing apparatus of the presentembodiment is described. FIG. 12 schematically illustrates drivingpulses used in the present embodiment. FIG. 12 illustrates drivingpulses supplied to a control line Tx1, a control line Tx2, and a controlline OFG of pixels on the n-th to the (n+2)th rows. The driving pulsessupplied to the control line Tx1, the control line Tx2, and the controlline OFG are the same as those in the first embodiment or in the thirdembodiment. In a case in which a pixel is not provided with a dischargeswitch 18, the driving pulses supplied to the control line OFG areunnecessary.

A corresponding transistor or a corresponding switch is turned ON whenthe driving pulses are at a high level. A corresponding transistor or acorresponding switch is turned OFF when the driving pulses are at a lowlevel. A control unit provided in the image capturing apparatus suppliesthese driving pulses. A logical circuit, such as a shift register and anaddress decoder, is used as the control unit.

In the present embodiment, the first transfer switch 4 is turned OFF ata part of a second period. Specifically, at time T6, the first transferswitch 4 is controlled from ON to OFF. Then, at time T7, the firsttransfer switch 4 is controlled from OFF to ON. With this configuration,a period in which the first transfer switch 4 is turns ON may beshortened. Therefore, noise generated in the first transfer switch 4 maybe reduced.

In the present embodiment, the first transfer switch 4 is againcontrolled from OFF to ON at time T8. Thus, control of the firsttransfer switch 4 to ON from OFF is performed a plurality of times inthe second period. With this configuration, noise may further bereduced.

It is desirable that the number of times of control of the firsttransfer switch 4 to ON from OFF is the same as or greater than a ratioof saturation charge quantity of the accumulation unit 2 to saturationcharge quantity of photoelectric conversion unit 1. In the presentembodiment, a ratio of saturation charge quantity of the accumulationunit 2 to saturation charge quantity of the photoelectric conversionunit 1 is 4. Therefore, control of the first transfer switch 4 to ONfrom OFF is performed four times in the second period.

As described above, according to the present embodiment, in addition tothe effect of the first embodiment, noise may be reduced.

Seventh Embodiment

Still another embodiment is described. The present embodiment differsfrom the first embodiment in the driving method. Thus, only a differencefrom the first embodiment is described and description of the sameconfiguration as that of the first embodiment is omitted.

Equivalent circuits of the present embodiment are the same as those ofthe first embodiment. FIG. 1 illustrates equivalent circuits of pixelsof the image capturing apparatus of the present embodiment. Descriptionabout FIG. 1 is the same as that of the first embodiment and is thusomitted.

A sectional structure of a pixel of the present embodiment is the sameas that of the first embodiment. That is, FIG. 2 schematicallyillustrates a sectional structure of a pixel of the present embodiment.Description about FIG. 2 is the same as that of the first embodiment andis thus omitted.

FIG. 13 schematically illustrates an operation of the image capturingapparatus of the present embodiment. FIG. 13 illustrates an image pickupoperation of from the n-th frame to the (n+1)th frame. An operationabout the n-th frame is illustrated by a solid line and an operationabout the (n+1)th frame is illustrated by a dotted line. FIG. 13illustrates an exposure period in each frame, a period in which thephotoelectric conversion unit 1 accumulates charge, and a period inwhich the accumulation unit 2 accumulates charge. FIG. 13 furtherillustrates a reading operation. The reading operation in FIG. 13 is anoperation including transfer of charge by the second transfer switch 5and output of a signal by the amplifier unit 10 which are described withreference to FIGS. 3 and 4. In FIG. 13, the first period and the secondperiod are equal in length.

As illustrated in FIG. 13, a plurality of times of reading operations ofpixels are performed after corresponding exposure periods end, that is,after the first period and the second period elapse. No charge isaccumulated in a period in which reading operation is performed. In thepresent embodiment, the exposure period is the sum of the first periodand the second period.

In such an operation, it is desirable that a ratio of saturation chargequantity A₁ of the photoelectric conversion unit 1 to saturation chargequantity A₂ of the accumulation unit 2 is substantially equal to a ratioof the first period P1 to the sum of the first period P₁ and a secondperiod P₂. That is, it is desirable that A₁, A₂, P₁ and P₂ satisfyExpression (5):

$\begin{matrix}{\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} \leq {\frac{P_{1} + {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}.}} & (5)\end{matrix}$

For example, in the present embodiment, the period in which thephotoelectric conversion unit 1 accumulates charge, i.e., the firstperiod, is half the length of one exposure period. Then, saturationcharge quantity of the accumulation unit 2 may be about twice as largeas saturation charge quantity of the photoelectric conversion unit 1.This is because the charge may overflow in the photoelectric conversionunit 1 even if the accumulation unit 2 has saturation charge quantitygreater than that. Therefore, by setting the ratio of saturation chargequantity as expressed by Expression (5), the optimum size of thephotoelectric conversion unit 1 and the accumulation unit 2 may bedetermined.

Eighth Embodiment

An embodiment of an image capturing system according to the presentdisclosure is described. Examples of the image capturing system mayinclude a digital still camera, a digital camcorder, a copier, afacsimile machine, a mobile phone, an in-vehicle camera, and anobservation satellite. Examples of the image capturing system may alsoinclude a camera module provided with an optical system, such as a lens,and an image capturing apparatus. FIG. 14 is a block diagram of adigital still camera as an example of the image capturing system.

In FIG. 14, the reference numeral 1001 denotes a barrier for protectinga lens, the reference numeral 1002 denotes a lens for forming an opticalimage of a subject on an image capturing apparatus 1004, and thereference numeral 1003 denotes a diaphragm for changing light quantitythat has passed through the lens 1002. The reference numeral 1004denotes the image capturing apparatus described in each of the aboveembodiments, which converts an optical image formed by the lens 1002into image data. Here, an AD conversion unit is provided in thesemiconductor substrate of the image capturing apparatus 1004. Thereference numeral 1007 denotes a signal processing unit that performsvarious correction and data compression to captured image data outputfrom the image capturing apparatus 1004. In FIG. 14, the referencenumeral 1008 denotes a timing generation unit that outputs varioustiming signals to the image capturing apparatus 1004 and to the signalprocessing unit 1007, and the reference numeral 1009 denotes an entirecontrol unit that controls the entire digital still camera. Thereference numeral 1010 denotes a frame memory unit that temporarilystores the image data, the reference numeral 1011 denotes an interfaceunit that performs recording or reading to a recording medium, and thereference numeral 1012 denotes a removable recording medium, such assemiconductor memory, in which the captured image data is recorded orfrom which the imaging data is read. The reference numeral 1013 is aninterface unit for communicating with, for example, an externalcomputer. Here, the timing signals or other signals may be input fromthe outside of the image capturing system, and it is only necessary thatthe image capturing system is at least provided with the image capturingapparatus 1004, and the signal processing unit 1007 that processes imagepickup signals output from the image capturing apparatus 1004.

In the present embodiment, a configuration in which the image capturingapparatus 1004 and the AD conversion unit are formed on the samesemiconductor substrate is described. However, the image capturingapparatus 1004 and the AD conversion unit may be provided on differentsemiconductor substrates. The image capturing apparatus 1004 and thesignal processing unit 1007 may be formed on the same semiconductorsubstrate.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the disclosure is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-052328, filed Mar. 14, 2014 which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image capturing apparatus, comprising: aplurality of pixels, each pixel including a photoelectric conversionunit configured to generate charge in response to incident light andaccumulate the charge, an accumulation unit configured to accumulate thecharge, an amplifier unit configured to output a signal based on thecharge, a first transfer switch configured to transfer the charge to theaccumulation unit from the photoelectric conversion unit, and a secondtransfer switch configured to transfer the charge to the amplifier unitfrom the accumulation unit; and an output line to which signals from theplurality of pixels are output, wherein at a first time point, thephotoelectric conversion units of the plurality of pixels startaccumulation of the charge, the first transfer switches of the pluralityof pixels are kept OFF from the first time point to a second time point,and the photoelectric conversion units of the plurality of pixelsaccumulate charge generated in a first period from the first time pointto the second time point, in a second period from the second time pointto a third time, the accumulation units of the plurality of pixelsaccumulate charge generated by the photoelectric conversion units in thefirst period, and charge generated by the photoelectric conversion unitsin the second period, at the third time, the first transfer switches ofthe plurality of pixels are controlled from ON to OFF, and saturationcharge quantity A₁ of the photoelectric conversion unit, saturationcharge quantity A₂ of the accumulation unit, the first period P₁, andthe second period P₂ satisfy the following relationship:$\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} \leq {\frac{P_{1} + {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}.}$2. The image capturing apparatus according to claim 1, wherein each ofthe plurality of pixels includes a discharge switch configured todischarge the charge in the photoelectric conversion unit; and from thefirst time point to the second time point, the discharge switch of atleast one of the plurality of pixels is kept OFF.
 3. The image capturingapparatus according to claim 2, wherein the accumulation of the chargeis started by controlling the discharge switch from ON to OFF.
 4. Theimage capturing apparatus according to claim 1, wherein the accumulationof the charge is started by controlling the first transfer switch fromON to OFF.
 5. The image capturing apparatus according to claim 1,wherein: the first transfer switches of the plurality of pixels areturned ON by the second time point at the latest; and in a part of thesecond period, the first transfer switches are turned OFF.
 6. The imagecapturing apparatus according to claim 5, wherein, in the second period,the first transfer switches are controlled from OFF to ON a plurality oftimes.
 7. The image capturing apparatus according to claim 6, whereinthe plurality of times is greater than a ratio of saturation chargequantity of the accumulation unit to saturation charge quantity of thephotoelectric conversion unit.
 8. The image capturing apparatusaccording to claim 1, wherein, from the second time point to the thirdtime, the first transfer switches of the plurality of pixels are keptON.
 9. The image capturing apparatus according to claim 1, wherein theaccumulation unit includes a first semiconductor region of firstconductivity type in which the charge is accumulated.
 10. The imagecapturing apparatus according to claim 9, wherein the accumulation unitincludes a second semiconductor region of second conductivity typedisposed on the first semiconductor region.
 11. The image capturingapparatus according to claim 9, wherein: the accumulation unit includesa third semiconductor region of second conductivity type disposed belowthe first semiconductor region, and a fourth semiconductor region ofsecond conductivity type disposed below the third semiconductor region;and impurity concentration of the third semiconductor region is higherthan impurity concentration of the fourth semiconductor region.
 12. Theimage capturing apparatus according to claim 1, further comprising awaveguide disposed to correspond to each of the photoelectric conversionunits of the plurality of pixels.
 13. The image capturing apparatusaccording to claim 1, wherein, in the first period, the second transferswitches of the plurality of pixels are turned ON in turns, and theamplifier units of the plurality of pixels output the signals to theoutput line.
 14. The image capturing apparatus according to claim 13,wherein, in the first period, the first transfer switches are controlledfrom OFF to ON in turns beginning from a pixel of which output of thesignal by the amplifier unit is completed.
 15. The image capturingapparatus according to claim 1, wherein, before the first time point,the second transfer switches of the plurality of pixels are turned ON inturns, and the amplifier units of the plurality of pixels output thesignals to the output line.
 16. The image capturing apparatus accordingto claim 1, wherein the first transfer switches of the plurality ofpixels are controlled from OFF to ON at the second time point.
 17. Theimage capturing apparatus according to claim 1, wherein saturationcharge quantity A₁ of the photoelectric conversion unit, saturationcharge quantity A₂ of the accumulation unit, the first period P₁, andthe second period P₂ satisfy the following relationship:$\frac{A_{1}}{A_{2}} = {\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)}.}$18. The image capturing apparatus according to claim 1, whereinsaturation charge quantity A₁ of the photoelectric conversion unit,saturation charge quantity A₂ of the accumulation unit, the first periodP₁, and the second period P₂ satisfy the following relationship:$\begin{matrix}{\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} < {\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)}.}} & (3)\end{matrix}$
 19. The image capturing apparatus according to claim 1,wherein saturation charge quantity A₁ of the photoelectric conversionunit, saturation charge quantity A₂ of the accumulation unit, the firstperiod P₁, and the second period P₂ satisfy the following relationship:$\begin{matrix}{\frac{P_{1}}{\left( {P_{1} + P_{2}} \right)} < \frac{A_{1}}{A_{2}} \leq {\frac{P_{1} - {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}.}} & (4)\end{matrix}$
 20. The image capturing system, comprising: the imagecapturing apparatus according to claim 1; and a signal processingapparatus configured to processes a signal from the image capturingapparatus.
 21. An image capturing apparatus, comprising: a plurality ofpixels, each pixel including a photoelectric conversion unit configuredto generate charge in response to incident light and accumulate thecharge, an accumulation unit configured to accumulate the charge, anamplifier unit configured to output a signal based on the charge, afirst transfer switch configured to transfer the charge to theaccumulation unit from the photoelectric conversion unit, and a secondtransfer switch configured to transfer the charge to the amplifier unitfrom the accumulation unit; and an output line to which signals from theplurality of pixels are output, wherein at a first time point, thephotoelectric conversion units of the plurality of pixels startaccumulation of the charge, the first transfer switches of the pluralityof pixels are kept OFF from the first time point to a second time point,and the photoelectric conversion units of the plurality of pixelsaccumulate charge generated in a first period from the first time pointto the second time point, in a second period from the second time pointto a third time, the accumulation units of the plurality of pixelsaccumulate charge generated by the photoelectric conversion units in thefirst period, and charge generated by the photoelectric conversion unitsin the second period, at the third time, the first transfer switches ofthe plurality of pixels are controlled from ON to OFF, and an area A₁ ofthe photoelectric conversion unit when viewed in a planar view, an areaA₂ of the accumulation unit when viewed in a planar view, the firstperiod P₁, and the second period P₂ satisfy the following relationship:$\frac{P_{1} - {0.5 \times P_{1}}}{\left( {P_{1} + P_{2}} \right)} \leq \frac{A_{1}}{A_{2}} \leq {\frac{P_{1} + {0.5 \times P_{2}}}{\left( {P_{1} + P_{2}} \right)}.}$